Normal operating conditions of photovoltaic (PV) conversion systems are rarely optimal. Depending on geographical location, season, and time of the day, sunshine conditions, partly or fully forecast skies will from time to time prevail. The spectrum of incident light differs considerably as a function of longitude and time of the day, from AM 1 to about AM 10. Under sunshine conditions, the operating temperature of a photovoltaic module, more so in hot climates or seasons, may be much higher than 25° C., set as the standard reference operating temperature, reaching values as high as 60° C. and even 80° C. This considerably lowers conversion efficiency of the cells, the open circuit voltage and to a lesser extent also the fill factor depends on the temperature of operation. These behaviors are parameterized by defining temperature coefficients dVoc/dT and dFF/dT, respectively. These coefficients differ for different photovoltaic active materials, though in general they are negative. By contrast, a relatively small positive temperature coefficient dJsc/dT of the short circuit current may be exhibited by most of the common materials used for the cells.
FIG. 1 shows typical I-V characteristics of a 15×15 cm2 crystalline silicon solar cell measured at STC, with indications of the performance parameter values. In particular, the so-called “fill factor” (FF) is a measure of the squareness of the I-V characteristics and is defined as FF=VmppImpp/VocIsc, and, in the shown sample is equal to 0.7111, that is to say that 71.11% of the area subtended by the characteristic curve from (0,0) to (Voc, Isc) is filled.
State-of-the-art, silicon based, photovoltaic panels normally reach an efficiency of about 15%.
An approach to increasing conversion efficiency is to use optical concentrators of the solar light, often associated with wavelength light splitters. These are for illuminating with the split beams, at each of the wavelengths in a specific region of the solar spectrum, “lambda-specific” photovoltaic cells made of active photovoltaic material most adapted to the specific range of wavelengths of the split beams.
Generally, this known class of light concentration photovoltaic cell systems assumes a relatively compact architecture because of the focusing of collected solar light, and with wavelength splitting and illumination with the split beams for distinct arrays of photovoltaic cells made with materials optimized for the specific wavelength range (lambda-specific).
Compactness, for reducing the length of light distribution paths to typically high power rating, highly efficient lambda-specific cells, and the need to seal off the light path from atmospheric fouling agents, may be severely detrimental to heat dissipation.
Heat dissipation restrictions may be addressed by employing distributed optical light concentration devices (lenses, reflectors) that are organized, for example, in a bi-dimensional array as the underlying array of photovoltaic cells. However, these alternative architectures often substantially weigh against deploying wavelengths splitters and lambda-specific photovoltaic cells. An extended bi-dimensional and multi-tier modularity generally complicates fabrication and assembly of distinct optically coupled functional components, requires numerous sun tracking actuators and related motorized structures, and has other drawbacks in terms of exposed area requirements, visual impact and impacts related to sensitivity from fouling of surfaces of optically coupled mirrors.